Peers are increasingly recognized as an important asset in healthcare. Peer supports are historically most prominent for substance use disorder (SUD). For trauma and posttraumatic stress disorder (PTSD), there have been very few attempts at peer help. A key challenge of trauma peer groups is that they usually focus on telling the story of members' trauma histories, which can be overly triggering and distressing. Seeking Safety (SS) offers an excellent choice for peer-led care. It is an evidence-based, widely adopted model for PTSD/SUD. It is extremely well-suited to peer delivery because it is highly stabilizing, structured, optimistic, and educational; and it has already been successfully implemented in peer-led format for many years by many different entities. We propose a mobile app for delivery of peer-led Seeking Safety (PSS) in this Phase I application. The app would be highly innovative: the first app for any trauma/SUD model, the first for trauma/SUD peer support, and the first for PSS. It has no existing competitors on the market. Given the high rate of trauma/SUD comorbidity and its far-reaching impact, such an app could have major public health importance as well as strong commercial potential. Our aims are as follows. (1) To develop the peer-led Seeking Safety (PSS) mobile app. This will be an exciting, engaging Android app for smartphones and tablets. We emphasize three app domains for end-users to access as well as two key processes that apply across the domains (all described in detail in the proposal, including user experience and technical details). Overall, we would leverage technology to create PSS in an online environment that would support recovery anytime, anywhere, powered by highly engaging content and delivery. Most people with trauma/SUD do not obtain formal treatment. The PSS app could address the need for access in remote locations, for anonymous help, and for convenient help, delivered by people who share their experiences as peers. Even for those who do attend in-person modalities, the app can provide additional support and unique features. For development of the app we emphasize an iterative process that includes focus groups with end-users as well as input from our outstanding consulting team, several of whom are peers themselves and/or have conducted PSS. (2) To conduct a feasibility study of the app. This pilot will obtain information on end-users experience, usage, and satisfaction with the app, which can inform a Phase II study if warranted by our Phase I results. Qualitative and quantitative data will be obtained pre and post, with an emphasis on validated instruments. Our truly stellar team has expertise in peer services; research on trauma/PTSD and SUD; telehealth; mobile app development for healthcare; and a focus on innovative products to improve care for vulnerable populations. This project would create a novel technology solution to provide care for an important population (people with trauma/SUD). The use of PSS in app form can create major impact from both a public health and commercial standpoint.